Bar spoons and other tools of the bartender’s craft are specialized instruments with designs that have evolved over centuries. The long, narrow spoons used to stir cocktails sometimes have spiral shafts. Why? Master (and, in fact, champion) bartender Kaitlyn Stewart explains that the spiral shaft helps bartenders pour layers into cocktails—especially carbonated drinks. The surface helps carbonated drinks retain that carbonation and not fizz over.
I’m going to guess that has something to do with the increased surface area that a spiral necessarily has. The adhesion of the water to the spoon would help control the descent of the liquid into the glass.
-via Laughing Squid